Projection using electrically charged particles, especially ions, can be used to form an image of the structure of a mask upon a substrate, e.g. a chip which can be utilized in the electronics industry. High accuracy is required in such systems and, for example, a plurality of exposures must be made through respective masks which must be superimposed upon one another with a high degree of precision so that the images formed on the substrate will be superimposed with high accuracy as well. The requirements for such projection systems in terms of tolerances and the like are thus high.
Nevertheless, there is a significant possibility that structure errors will be transmitted to the substrate, such errors arising from a variety of sources. For example, there may be errors in the structure of the mask whose image is to be imparted to the substrate. A typical fault of this type can include a line of the structure which is not precisely straight, e.g. has a certain curvature. Other errors in forming the image can also be introduced and it is desirable to provide a projection system and process which is capable of eliminating these errors.
A starting point for such correction is, of course, the detection of the error, at the image side of the system, so that changes can be made in the projection system to rectify the error.
It is known, in this connection, to provide an electronic shadow exposure system in which a divergent electron beam from an electric source is trained upon a surface formed with a hexagonal opening. By means of a collimator lens which surrounds the opening , a parallel beam can be generated which extends along the axis of the optical column.
In a so-called deflection region, two deflection systems are provided. By means of a first deflection system, the beam is scanned parallel to the beam axis transversely over the mask surface. By means of a second deflection system, the beam is tilted about a deflection axis lying in the mask plane so that it impinges upon the mask at an angle offset by 90.degree. from the original beam axis, thereby dynamically shifting the image of the mask opening or individual images of the edge of the overall structure.
It is thus possible to form images on the substrate which are shifted by small amounts by tilting of the beam directions during scanning, from the image positions which might otherwise result without this tilting action.
If one applies a constant tilt to the scanning beam, it is possible to shift the entire mask image on the substrate.
Utilizing this principle, it is possible to compensate for position deviations of the table which carries the substrate. If one also changes the tilt of the beam during the scanning operation, it is also possible to achieve a slight degree of rotation of the image to compensate for errors for table vibration or the like.
The known system for this purpose also permits, by a corresponding tilting of the beam, a modification of the image in nonlinear manner during the shadow exposure and this permits compensation of mask distortion during substrate exposure.
Notwithstanding the fact that significant compensation of errors and distortion is possible with earlier systems, the apparatus involved and its manipulation is relatively complicated.